The present invention relates to an epitaxial wafer for fabricating a high-intensity infrared light-emitting device which is employed in an optical communications and spatial transmission apparatus using infrared radiation. The invention also relates to an infrared light-emitting device employing the epitaxial wafer and an optical communications and spatial transmission apparatus employing the device.
Light-emitting devices employing a Ga1-XAlXAs (wherein 0xe2x89xa6X less than 1) (hereinafter abbreviated as GaAlAs) compound semiconductor have been widely used in a light source in a wavelength range from infrared to visible red light. Although an infrared LED is employed in optical communications and spatial transmission, there has been increasing demand for a high-intensity infrared LED of increased capacity for transmitting data and increased transmission distance.
As has conventionally been known, a GaAlAs LED is produced, for example, by forming semiconductor films through epitaxial growth on a GaAs substrate. Such a GaAlAs LED having a double-hetero structure (hereinafter DH structure) exhibits emitted-light intensity higher than that of a GaAlAs LED having a single-hetero structure, and emitted-light intensity is enhanced by means of removing a substrate.
In fabrication of an LED employing a substrate-removed-type structure (hereinafter referred to as a DDH structure), a typical DH structure; i.e., only three layers consisting of a p-type cladding layer, an active layer, and an n-type cladding layer, is epitaxially grown and then a substrate is removed, to thereby reduce the thickness of a produced epitaxial wafer. Such an epitaxial wafer is difficult to handle during processing into a device. In addition, a paste for bonding the device to a conductor migrates through a side face of the device, to thereby disadvantageously short-circuit the pn junction. In order to avoid this problem, a fourth epitaxial layer is added to the DH structure so as to ensure the overall thickness of the substrate-removed and finished epitaxial wafer and the distance from a bottom surface of the device to the junction. This constitution is standard for a DDH structure. The fourth epitaxial layer is designed to have a band gap wider than that of an active layer so as not to absorb emitted light from the active layer. The fourth epitaxial layer may be added on the n-type cladding layer or on the p-type cladding layer of the aforementioned DH structure. In addition, the fourth epitaxial layer may be formed singly or in combination with a plurality of epitaxial layers.
The present inventors have conducted earnest studies in order to enhance emitted-light intensity of and lower variation in emitted-light intensity of an epitaxial wafer having a DDH structure for fabricating an infrared LED and an infrared LED fabricated from the epitaxial wafer, and have found the following relationships existing during stacking steps for producing an epitaxial wafer comprising a first p-type layer, a p-type cladding layer, a p-type active layer, and an n-type cladding layer.
Specifically, sulfur atoms introduced into the n-type cladding layer lower emitted-light intensity, and the decrease in intensity induced by sulfur atoms in the n-type cladding layer is closely related with the thickness of the n-type cladding layer.
In an initially grown portion of the p-type cladding layer, high-concentration impurities may be segregated at the interface between the p-type cladding layer and the first p-type layer. The segregation lowers emitted-light intensity of the LED and induces variation in emitted-light intensity.
Among the impurities contained in an initially grown portion of the p-type cladding layer, silicon provides the most significant adverse effect. The maximum silicon concentration in the portion of the p-type cladding layer within 2 xcexcm of the interface between the p-type cladding layer and the first p-type layer is controlled to less than 1xc3x971018 atoms/cm3, to thereby remarkably enhance emitted-light intensity of the LED and lower variation in emitted-light intensity.
Enhancement of emitted-light intensity of the LED is closely related with the impurity concentrations and the carrier concentration in the first p-type layer. Among the impurities, carbon, sulfur, and oxygen have remarkable adverse effects. When the thickness of the p-type cladding layer is 50-80 xcexcm; i.e., the optimum range, emitted-light intensity of the LED is enhanced. The first p-type layer preferably has a carrier concentration of 3xc3x971017 to 1xc3x971018 cmxe2x88x923.
When Ge is introduced as a dopant into the p-type active layer, a negative correlation is found between Ge concentration in an n-type GaAlAs layer and intensity of emitted light. The present invention has been accomplished on the basis of these findings. Accordingly, the present invention is directed to
[1] an epitaxial wafer for fabricating an infrared light-emitting device, which wafer is produced by sequentially forming on a p-type GaAs single-crystal substrate a first p-type layer (Ga1-X1AlX1As, 0.13xe2x89xa6X1xe2x89xa60.40); a p-type cladding layer (Ga1-X2AlX2As, 0.23xe2x89xa6X2xe2x89xa60.46); a p-type active layer (Ga1-X3AlX3As, 0xe2x89xa6X3xe2x89xa60.03) having an emission wavelength of 850-900 nm; and an n-type cladding layer (Ga1-X4AlX4As, 0.13xe2x89xa6X4xe2x89xa60.40) through liquid-phase epitaxy and removing the p-type GaAs single-crystal substrate, wherein the n-type cladding layer has a carrier concentration of 1xc3x971017 to 1xc3x971018 cmxe2x88x923 and a sulfur concentration of 3xc3x971016 atoms/cm3 or less;
[2] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1], wherein the n-type cladding layer has a thickness of 20-50 xcexcm;
[3] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the maximum silicon concentration in the portion of the p-type cladding layer within 2 xcexcm of the interface between the p-type cladding layer and the first p-type layer is less than 1xc3x971018 atoms/cm3;
[4] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the concentration of carbon, sulfur, or oxygen in the first p-type layer is less than 1xc3x971017 atoms/cm3;
[5] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the p-type cladding layer has a thickness of 50-80 xcexcm;
[6] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the first p-type layer has a carrier concentration of 3xc3x971017 to 1xc3x971018 cmxe2x88x923;
[7] an epitaxial wafer for fabricating an infrared light-emitting device as described in [1] or [2], wherein the p-type active layer contains germanium as a predominant impurity and the n-type cladding layer contains germanium at a concentration of 3xc3x971018 cmxe2x88x923 or less;
[8] a light-emitting device fabricated by use of an epitaxial wafer for fabricating an infrared light-emitting device as recited in [1] or [2]; and
[9] an optical communications and spatial transmission apparatus employing a light-emitting device as recited in [8].